The present invention relates to turbine vane rings, particularly ceramic vane rings for gas turbines composed of a plurality of vanes extending between radially inner and outer cylindrical cover rings.
It is known that the efficiency and the specific output of a gas turbine are related to the temperature of the working gases. Since the use of metallic materials for the gas-carrying components of a gas turbine limits the attainable gas temperatures by the relatively low heat resistance of metallic materials, it is recognized that the working temperatures, and thereby the efficiency and the specific output, of a gas turbine can be increased by making use of ceramic materials, such as silicon nitride, silicon carbide or aluminum titanate for high temperature components, such as the stator vane ring.
Difficulties have been encountered in fabricating such components from ceramic materials, since in addition to good aerodynamics of the flow paths, such components must have good manufacturing capability and high stability relative to the thermal stresses occurring at extreme operating conditions. It has been found that the vane rings of gas turbines are subjected to considerable heat stresses, particularly during the starting phase, and after shut down of the gas turbine from full load. One zone undergoing particular stress is the junction of the vanes and the outer cover ring, specifically in the vicinity of the axially trailing edges of the vane. These stresses can easily result in fractures, and can assume especially high values where vane rings are formed of a single piece, which is desirable from the point of view of leakage and integration into a gas turbine plant. Since such a monolithic vane ring, i.e., a vane ring consisting of one piece, is difficult to produce from ceramic materials, a method has become known whereby the vane ring is formed by connecting individual segments, each containing a vane and corresponding portions of the inner and outer cover rings. Such segments can be fabricated individually and subsequently held together by special means, such as bandages and the like. This type of vane ring construction necessitates a considerable effort in components and construction space.
It is, therefore, a purpose of the present invention to create a vane ring composed of ceramic materials for thermal flow machines, in particular for gas turbines, which offers an optimal design as regards heat stresses during rapidly changing operating conditions, as well as better aerodynamics, manufacturing capability and ease of fabrication.